1. Field of the Invention
This invention relates to a frame unit which forms a part of an optical system movement type exposure device to be used in an image forming apparatus such as an electrophotographic printing machine or printer.
2. Description of the Related Art
Applicant has filed an unexamined, published Japanese Patent Application No. 116664/1989 directed to an optical system moving frame unit which is substantially free from twisting vibration or vertical vibration. A reduction in vibration reduces the chances that the formation of images will be adversely affected by external exciting forces. This optical system moving frame unit economically utilizes space and reduces the weight of the exposure device.
The optical system moving frame unit is made up of a rectangular frame reinforced with reinforcing ribs arranged as in a trussed structure. A rectangular frame wastes less space than a trapezoidal frame. The reinforcing ribs increase the natural frequency of the optical system moving frame unit. A higher natural frequency substantially decreases the twisting vibration or vertical vibration attributing to external exciting forces.
However, the optical system moving frame unit suffers from the following problems when it is produced by large scale casting. Molten aluminum does not flow smoothly into the cavities for the reinforcing ribs in the casting operation, so blowholes are likely to be formed in the resultant reinforcing ribs. Therefor, some of the reinforcing ribs are not completely formed because the cavities for the reinforcing ribs are not sufficiently filled with the molten aluminum.